Exercise experts have known for years that moderate physical activity improves mood. Now a study shows that people are in a better mood for up to 12 hours after they work out.

Researchers at the University of Vermont had 24 college students ride an exercise bike for 20 minutes at a moderate intensity. Another 24 people did no exercise during the same time period.

Afterward, everyone in both groups filled out questionnaires that evaluated their overall mood at several intervals: one hour, two, four, eight, 12 and 24 hours. The questions addressed tension, anger, vigor, fatigue, confusion and depression.

The researchers found that people in the exercise group experienced a significant improvement in mood immediately after the exercise. They also had improved moods after two, four, eight and 12 hours compared with the people who didn't exercise, says Jeremy Sibold, an assistant professor in the university's Department of Rehabilitation and Movement Science.

At 24 hours, there was no difference in mood between the two groups. The study was presented last week at the annual meeting of the American College of Sports Medicine.

"There are other studies that show there is acute improvement in mood after exercise, but we found those improvements are more durable than we thought," Sibold says.

That's an especially important finding for people who have depression or stress, he says.

"Daily exercise can improve your mood and mitigate some of the stressors of your day," Sibold says. "It's clear that exercise is critical for both physical health and mental health."